Federal Rule of Evidence 803 sets out three hearsay exceptions for spontaneous events: present sense impression, excited utterance, and then-existing condition. The rationale for these exceptions is that the spontaneous statements are reliable because a person is unlikely to fabricate lies (which presumably take deliberate reflection) while her mind is preoccupied with the stress of an exciting event.
But for lots of complicated reasons explained here (click the link for a primer on peritraumatic dissociation and elevated cortisol levels in the amygdala), that’s not how the brain works. First, “it's not true that people can't make up a lie in a short period of time. Most lies in fact are spontaneous.” Second, the emotional stress attending a crime can fragment and corrupt memories. The assumptions behind the spontaneous event hearsay exceptions are exactly backward. Spontaneity does not enhance reliability; it may compromise it.
Courts are beginning to recognize the fallacy of the rationale behind the spontaneity hearsay exceptions. “As with much of the folk psychology of evidence, it is difficult to take this rationale entirely seriously, since people are entirely capable of spontaneous lies in emotional circumstances…It is time the law began paying attention to such studies.” So, if the government’s case relies on a spontaneity exception, ask the court to view that evidence in light of the science.